The Internet Protocol Multimedia Subsystem, IMS, is an architectural framework for delivering IP multimedia services to a user. In addition, the industry has identified a need for detailed standardization of key IMS-based services, in particular for person-to-person communication. ETSI TISPAN (Telecoms & Internet converged Services & Protocols for Advanced Networks) has thus initiated the standardization of IMS-based multimedia telephony services (MMTel). 3GPP standard 22.173 Version 12.5.0 defines the MMTel service and the minimum set of capabilities required to secure multi-vendor and multi-operator inter-operability for Multimedia Telephony and related Supplementary Services. MMTel enables people to communicate using any combination of voice, video, and real-time text. It also defines a set of supplementary and regulatory services. An MMTel communication can start with only one type of media and additional types of media may or may not be added by the users as the communication progress.
Another possibility in IMS to add a supplementary service is the use of the Video Share service in IMS (also called as a Video share service, specified by GSMA). This service is used to allow users which are engaged in a Circuit-Switched, CS, voice call to add a unidirectional video streaming session over the packet network during the voice call. Any of the parties on the CS voice call can initiate a video streaming session. This Video Share service requires a 3GPP compliant IMS core system and is described is 3GPP standard document 22.279 Version 11.0.0 and GSMA official document IR. 74 and IR. 84. The Video Share service differs from a traditional Video Call service in that a Video Call involves simultaneous two-way Video and Audio transmission between the two parties (from start to finish), whereas the Video Share service involves adding and removing one or more one-way Video sessions to an existing voice call between the two parties. This new additional session, with any media, can be seen as a supplementary session as the communication endpoints are the same and the supplementary session is executed in parallel to the first established session. Therefore a Video Share session can be seen as an example of a supplementary session to the established CS voice call session.
Sometimes a calling user, which can be named as a remote user, has chosen to be anonymous to the called user, which can be named as a local user, so that the called user does not have the real identity of the calling anonymous user. It may also be possible that one user is connected behind a group number which is provided to the other participant of the session. If one user of a CS voice call session or an MMTel session has chosen to be anonymous or is using a group identity to the other user, it is not possible to establish a further session as a supplementary session because the identity of the user must be known to the user who would like to add a supplementary session, e.g. a Video Share Service.